Peter wrote:
The relative altitude is not being determined using the same reference – unless both “portable” boxes are using GPS altitude and then, assuming geoid correction is being done right…
The altitude broadcast by ADS-B is geometric altitude and does not include any corrections for the geoid, it is altitude with respect to the WGS-84 spheroid.
OK… I am no doubt not making my line of thinking very clear.
Also we have the two very different scenarios.
One is NCyankee’s very accurate posts on the US scenario, which is always “certified” ADS-B, SIL/SDA=3. And presumably any baro altitude being radiated will be “transponder accuracy”.
The other is the European (and especially the current UK) scenario, where most of the drive is towards portable boxes which can’t possibly be radiating “transponder accuracy” baro altitude but in a traffic warning scenario are potentially going to be receiving “transponder accuracy” baro altitude from a modern well equipped ADS-B OUT aircraft.
If using any form of a Baro Pressure altitude for comparison with a target aircraft, a portable system that does not use the same altitude as the transponder, is going to have more error, but I would expect it to be on the order of a 100 feet to maybe 300 feet. If the only purpose is to aid the pilot to visually acquire a nearby target, it is probably better than nothing, but one should include in their scan a greater area vertically to look for traffic. If one is comparing a geometric altitude verses a target with a pressure altitude, the two systems are measuring two totally different things and it is not a valid basis for relative altitude comparison.
Exactly, so what would e.g. a SkyEcho 2 show in relative altitude, when displaying an ADS-B target whose baro altitude is “transponder accuracy”?
Peter wrote:
How does that work in a cockpit, where the pressure is typically a few mb less than outside
In my experience, the difference has been under 50 feet (1.5mb). “A few” mb difference would probably indicate an aircraft of sufficient performance that it will have rather fancier avionics than needing a portable ADS-B transmitter.
Reading the SkyEcho 2 website information, it includes a baro altimeter that would sense cabin pressure altitude. If connected to ForeFlight, the ownship data provided by the SkyEcho 2 would be used and it includes the pressure altitude sensed from the cabin. So depending on the error between cabin altitude and actual pressure altitude, one would need to compensate for any difference. If you set the ships altimeter to standard 29.92 inches or 1013 MB, a pilot can compare the two in flight to gage how much error is added.
I see a 100ft delta at low level and some 200ft higher up, say FL120.
Yes, but you fly the kind of plane that tends to have fancier avionics.
Not so sure. The TB20 is no speed machine; lots of people will be looking at putting one of these portable gadgets into an RV etc etc.
Would a GTX335 or similar certified ADS-B OUT device be radiating both baro and GPS altitude?
If Yes then I am surprised that the receivers such as the Skyecho 2 don’t automatically select the GPS value and discard the baro value, if the GPS value is available and especially if the received signal is SIL=3 which is telling it that there is likely to be a significant baro altitude mismatch.
Peter wrote:
If Yes then I am surprised that the receivers such as the Skyecho 2 don’t automatically select the GPS value and discard the baro value, if the GPS value is available and especially if the received signal is SIL=3 which is telling it that there is likely to be a significant baro altitude mismatch.
I don’t understand the relevance of SIL.
